fix: clear all database tables on startup

src/database.py

@@ -44,6 +44,8 @@ def init_db():
         CREATE INDEX IF NOT EXISTS idx_processed_account ON processed_emails(account);
     """)
     conn.execute("DELETE FROM processed_emails")
+    conn.execute("DELETE FROM email_contexts")
+    conn.execute("DELETE FROM conversations")
     conn.commit()
     logger.info("数据库初始化完成: %s", DB_PATH)